In recent years, the desire to reduce environmental impact has resulted in growing demands for clean liquid fuels that contain minimal amounts of sulfur components and aromatic hydrocarbons and are gentle on the environment. As a result of these demands, processes that employ a Fischer-Tropsch synthesis reaction (hereinafter abbreviated as “FT synthesis reaction” in some cases), which uses carbon monoxide and hydrogen gas as feedstocks, have begun to be investigated as potential processes that are capable of producing fuel oil base stocks, and particularly kerosene and gas oil base stocks, that contain no sulfur components or aromatic hydrocarbons and are rich in aliphatic hydrocarbons, (for example, see Patent Document 1).
The synthetic oil (raw oil) obtained by the FT synthesis reaction (hereafter referred to as “FT synthetic oil” in some cases) is a mixture containing mainly aliphatic hydrocarbons having a broad carbon number distribution. From this FT synthetic oil can be obtained a naphtha fraction containing a large amount of components having a boiling point lower than approximately 150° C., a middle distillate containing a large amount of components having a boiling point within a range from approximately 150° C. to approximately 360° C., and a wax fraction containing a large amount of components that are heavier than the middle distillate (namely, components having a boiling point that exceeds approximately 360° C.). Of these fractions, the middle distillate is the most useful fraction, being equivalent to a kerosene and gas oil base stock, and it is desirable to achieve a high yield of this middle distillate. Accordingly, in an upgrading step in which the FT synthetic oil is hydroprocessed and fractionally distilled to obtain fuel oil base stocks, the wax fraction, which is produced in a reasonably large amount together with the middle distillate during the FT synthesis reaction step, is subjected to hydrocracking to reduce the molecular weight and convert the wax fraction to components equivalent to the middle distillate, thereby increasing the overall yield of the middle distillate.
Specifically, an example illustrated in FIG. 1 is given as a process for obtain the middle distillate by hydrocracking the wax fraction contained in the FT synthetic oil. That is, the crude wax fraction prior to hydrocracking, which is obtained from the FT synthetic oil by fractional distillation in a first fractionator 10, is supplied to a wax fraction hydrocracking device 50 via a line 14 and subjected to hydrocracking. The hydrocracked product is then supplied to a second fractionator 20 through a line 51. Meanwhile the crude middle distillate prior to hydrotreating, which is obtained from the FT synthetic oil by fractional distillation in the first fractionator 10, is supplied to a middle distillate hydrotreating device 40 via a line 13 and subjected to hydrotreating. The hydrotreated product then passes through a line 41, merges with the above-mentioned hydrocracked product, and is supplied to the second fractionator 20. The hydrocracked product and hydrotreated product supplied to the second fractionator 20 are fractionally distilled, and a middle distillate that becomes a kerosene and gas oil base stock is obtained from a line 22. Further, a bottom oil, composed mainly of a so-called “uncracked wax” that has not sufficiently undergone cracking during the wax fraction hydrocracking, is discharged from the bottom of the second fractionator 20, and this bottom oil is recycled through a line 24 to the line 14 that is upstream from the wax fraction hydrocracking device 50, and once again subjected to the hydrocracking.
The hydrocracked product discharged from the wax fraction hydrocracking device 50 includes not only hydrocarbon components having molecular weights that have fallen below a predetermined level as a result of the hydrocracking, but also the above-mentioned uncracked wax. This uncracked wax is a component having a high freezing point, and the hydrocracked product containing the uncracked wax is usually a solid or semisolid that exhibits no fluidity at a normal temperature and normal pressure.
Incidentally, when the second fractionator 20 is started up after a long period in a non-operational state, the second fractionator 20 and the lines and the like connected thereto are either at a normal temperature or a temperature close thereto.
If the above hydrocracked product of the wax fraction is supplied to the second fractionator 20 under these conditions, then the temperature of the hydrocracked product will fall, and there is a possibility that the hydrocracked product may become a solid or semisolid and block the second fractionator 20 or the lines and the like connected thereto. Accordingly, during startup of the fractionator 20, prior to supply of the hydrocracked product from the wax fraction hydrocracking device 50 to the fractionator 20, a hydrocarbon oil (hereinafter referred to as the “heating oil” in some cases) that is liquid at a normal temperature and normal pressure is introduced into the fractionator from an external source, and by heating and circulating this heating oil, the second fractionator 20 and the lines and the like connected thereto are preheated to a temperature that is sufficiently high to prevent the hydrocracked product from solidifying.
Following preheating of the second fractionator 20 in this manner, supply of the hydrocracked product from the wax fraction hydrocracking device 50 and the hydrotreated product from the middle distillate hydrotreating device 40 to the second fractionator 20 is started, and the second fractionator 20 begins operating. Employing this method enables prevention of the type of problem that occurs when those components contained within the hydrocracked product that have a high freezing point undergo cooling and solidification inside the second fractionator 20 or the lines and the like connected thereto, causing blockages within the device.
Following completion of the preheating of the second fractionator 20, the heating oil is transported to a slop tank via a discharge line 29 that branches from the line 24.